1. Field of the Invention
This invention relates to electrographic recording and, more particularly, to apparatus for stripping magnetically attractive developer from the surface of the magnetic brush applicator after being used to develop an electrostatic image.
2. Description of the Prior Art
In the commonly assigned U.S. Pat. No. 4,473,029, issued to Fritz et al, there is disclosed an electrographic development system comprising a magnetic brush applicator and a magnetic development mix. The applicator basically comprises a nonmagnetic cylindrical sleeve having a rotatably driven magnetic core piece positioned therein. The magnetic core piece is driven at a high speed (e.g. 1500 RPM). The magnetic developer comprises a mixture of thermoplastic toner particles and magnetically "hard" carrier particles, the latter having relatively high magnetic coercivity and remanence. Rotational movement of the applicator's core piece serves to transport the development mix over the sleeve's outer surface and into contact with an electrostatic image-bearing surface located at a development zone. While in contact with the image, the electrostatic forces associated with the image overcome the triboelectric attraction between toner and carrier, thereby stripping the toner from the carrier and depositing it on the image to effect development thereof. After development, the partially denuded carrier particles are returned to a reservoir to be replenished with toner particles.
In development systems of the above-mentioned type, some difficulty has been encountered in stripping the developer mix from the applicator's sleeve after development to effect toner replenishment. Owing to the relatively small size of the carrier particles (e.g. 20-40 microns) and their high magnetic affinity for the magnetic core piece located within the sleeve, it is necessary to physically scrape the development mix from the applicator's sleeve in order to achieve separation. For this purpose, it is generally known in the art to use a thin, flexible scraper blade, often referred to as a "skive". For example, U.S. Pat. Nos. 4,338,880 and 4,392,735 disclose skives made from thin strips (30-100 microns thick) of plastic or non-magnetic metal. Such flexible skives are commonly supported along one edge by a rigid member which is positioned to urge the free edge of the skive into contact with the periphery of the applicator's sleeve. Similar skiving devices are disclosed in U.S. Pat. Nos. 4,328,762 and 4,391,503.
Stripping skives of the type disclosed above are sometimes difficult to maintain in alignment and in uniform contact with the periphery of the magnetic brush's non-magnetic sleeve. This is particularly true when the sleeve is rotatably driven. Non-uniform contact produces non-uniform wear of the blade, producing worn spots or "steps" in which developer may become trapped. Moreover, non-uniform contact will produce differential frictional heating which can cause such trapped developer to agglomerate and form large "flakes" which adversely affect image quality.